Noise suppressor for strain gauges



June 29, 1954 c. M. HATHAWAY 2,682,170

NOISE SUPPRESSOR FOR STRAIN GAUGES Filed July 29, 1950 Patented June 29,1954 NOISE SUPPRESSOR FOR- STRAIN GAUGES Claude M. Hathaway, Denver,Colo., assignor to Hathaway Instrument Company, Denver, 0010., acorporation of Colorado Application July 29, 1950, Serial No. 176,579

8 Claims. 1

The present invention relates to an apparatus and circuit for reducingthe noise heretofore encountered in the use of strain gauges mounted onrotating members.

Frequently it is necessary to mount resistance or inductance straingauges on a rotating member and to make electrical connections thereto.It has been found to be the best practice to mount an entire bridgenetwork of gauges on the rotating member and to use a plurality of sliprings for connections to the bridge network. Normally two slip rings areused to supply alternating current excitation voltage to the bridge andtwo other slip rings are used to bring out the unbalanced voltage, whichthen may be utilized by any suitable circuit.

In such an arrangement, however, any changes in the slip ring brushcontact resistance do not produce an unbalance of the bridge and, hence,with a zero strain or zero bridge output, no output voltage is producedby changes in the contact resistance of the slip rings. If, however,there is an unbalanced voltage generated because of strain beingpresent, changes in contact resistance of the slip rings supplying thealternating current source will produce changes in output voltage. Tominimize this detrimental effect it has been proposed to utilize gaugesof high impedance so that the contact resistance change should becomparatively small with respect to the impedance of the bridge network.However, even with the use of high impedance gauges and networks,variations in slip ring contact resistance introduce troublesome noisesor undesired components in the output voltage.

t, therefore, would be desirable to provide a circuit or means whichwill minimize the effects of changes in slip ring contact resistance.

It is, therefore, an object of the present invention to provide animproved circuit and method for minimizing the detrimental eifectsheretofore encountered in slip ring brush contact resistance wherestrain gauges are mounted on rotating members.

A still further object of the present invention is to provide anarrangement whereby contact noise generated by slip rings may beminimized and whereby a correction factor may be intro-' duced havingthe same phase angle as that of the bridge network.

Other and further objects of the present invention subsequently willbecome apparent by reference to the following description taken inconjunction with the accompanying drawing wherein the circuit diagramillustrates the principles of the present invention.

In the drawing there is disclosed a strain gauge network, which might becomposed of wire strain gauges or inductance gauges. Four inductancegauges ll, l2, l3 and I4 have been represented as connected in a bridgenetwork and mounted on a rotating member. Two of the gauges are arrangedto be non-responsive to changes in stress or strain of the rotatingmember so that a change in the impedance of the strain responsiveelements will produce an unbalanced voltage at the output terminals ofthe bridge network, which are connected to slip rings 3 and 4. Two inputterminals to the bridge network are provided which are connected to sliprings l and 2. A suitable source of alternating current voltage I5 isprovided having the desired frequency, which might be several thousandcycles per second. One terminal leading from the source of alternatingcurrent voltage I5 is connected through a transformer winding I! to thecommutator brush of the slip ring I. The transformer is provided withanother winding l8 having one terminal connected to the source ofcurrent E5. The other terminal of the transformer winding I8 isconnected through an adjustable resistor or impedance I9 to thecommutator brush of the slip ring 2, which is also connected to theother terminal of the alternating current source IS. The transformer hasstill another winding 20 having one terminal connected to the commutatorbrush of the slip ring 3. Connected across the transformer winding 20 isa voltage divider resistor 21 having its movable contact connected toone conductor of the output terminals It. The other output terminal I6is connected to the commutator brush of the slip ring l. Where it isdesired to provide means for making adjustments so as to obtain the samephase angle for the compensating effects as that of the bridge network,a variable capacitor 22 is connected in parallel with the voltagedivider 2|.

The transformer winding I! is connected in series with the alternatingcurrent source l5 so as to produce a magnetic flux in the transformercore, which is proportional to the alternating current flowing throughthe transformer. The other winding I8 is connected in series with avariable impedance [9 across the alternating current source in suchmanner that the fluxes produced by the windings I I and I 8 oppose eachother. By adjusting the variable impedance 19, the flux produced in thecore of the transformer can be adjusted to zero.

If now it is assumed there is a change in the contact resistance of thebrushes with the slip rings, the current flowing through the winding llof the transformer will change and a voltage will be introduced in thesecondary winding 20. The secondary winding 2? is connected to a voltagedivider 2i so that a desired portion of this voltage is introduced inseries with the output voltage in order to balance out the voltagechange, which would be produced inthe output due to slip ring contactresistance.

If now it is assumed that none of the slip rings are turning, slip rings3 and 4 may be connected together and the resistor 59 is adjusted sothat no voltage is induced in the secondary winding 20. The connectionbetween the slip rings 3 and 4 is then removed and the slip rings arerotated. Thereupon the contact on the voltage divider 2| is adjusted forminimum contact noise. Since the voltage introduced into the outputcircuit must have the same phase angle as that obtained from the outputterminals of the bridge network, the eifective impedance may comprisethe resistor 2| and a variable capacitor 22. Where the use of acapacitor 22 has been found necessary, the capacitor 22 must be adjustedindependently to obtain the proper balance.

While for the purpose of illustrating and describing the presentinvention a certain desired circuit arrangement has been shown in thedrawing, it is to be understood that the invention is not to be limitedthereby, since such variations in the circuit and in the componentsemployed therein are contemplated as may be commensurate with the spiritand scope of the invention set forth in the'accompanying claims.

I claim as my invention:

1. A noise reducing arrangement for a strain responsive bridge networkmounted on a rotating member having input and output slip rings, networkhaving an input and an output connected to said input and output rings,respectively, comprising a source of alternating current connected tosaid input rings, a transformer having one winding connected betweensaid source of current and one of said input rings, means forneutralizing the flux produced in said transformer by current normallyflowing through said winding, and means connected in series with one ofthe output rings for deriving from said transformer when current flowingtherethrough changes a potential to be introduced in series with theoutput of said network.

2. A noise reducing arrangement for a strain responsive bridge networkmounted on a rotating member having input and output slip rings, saidnetwork having an input and an output connected to said input and outputrings respectively the output of said network to neutralize noiseeffects produced by variations of contact resistance at said slip rings.

3. A disturbance .reducing arrangement for a strain responsive bridgenetwork mounted on a rotating member having an input and an out put, aplurality of input and output slip rings, connections between said inputand output and said input and output slip rings respectively, a

source of alternating current connected to the input rings of saidnetwork, a transformer having one winding connected in series with oneof the input rings to said network, said transformer having anotherwinding arranged for neutralizing the flux produced in said transformerby current normally flowing throughsaid first winding, and meansconnected in series with one of the output rings for deriving from saidtransformer when current flowing therethrough changes a potential to beintroduced in series with the output of said network to neutralizedisturbances produced by variations of contact resistance at said sliprings.

4. The combination for eliminating the eifects of contact resistancevariations on input and output slip rings for a strain responsive bridgenetwork mounted on a rotating member, said network having an input andan output, comprising connections between the input and the output ofsaid network to said input and output slip rings respectively, a sourceof alternating current connected through said input slip rings to saidnetwork, means connected between said source and said network forproducing potentials in accordance withvariations of contact resistanceof said slip rings, and means connected in series with one of the outputrings for introducing into the output of said bridge network componentsof said potentials of such phase and magnitude as to neutralize theeffects of contact resistance variations.

5. The combination for eliminating the effects of contact resistancevariations on input and output slip rings for a strain responsive bridgenetwork mounted on a rotating member and having an input and an outputconnected to said input and output slip rings respectively comprisingasource of alternating current connected to the input rings to saidnetwork, transformer means connected in series with said source and oneof said input rings for producing potentials in accordance withvariations of contact resistance of said slip rings, and means connectedto said transformer for introducing in series with one of the outputrings of said bridge network components of said potentials of such phaseand magnitude as to neutralize the effects of contact resistancevariations.

6. The combination for a strain responsive bridge network mounted on arotating member provided with input and output slip rings, an input andan output for said network, comprising connections between the input andthe output of said network to said input and output slip rings,respectively, a source of alternating current, a transformer havingprimary, secondary and tertiary windings, said primary winding beingconnected in series between said alternating current source and one ofsaid input rings, a variable impedance, said impedance and. saidtertiary winding being connected in series across said source ofalternating current, said secondary winding being connected in serieswith the output rings of said bridge network.

7. The combination for a strain responsive bridge network mounted on arotating member having input and output slip rings connected to theinput and the output of said network respectively comprising a source ofalternating current,

a transformer having primary, secondary and tertiary windings, saidprimary winding being connected in series between said alternatingcurrent source and one of said input rings, a variable impedanceconnected in series with said tertiary winding and said source ofalternating current, a phase shifting network, said secondary windingbeing connected in series with said phase shift ing network and one ofthe output rings of said bridge network.

8. The combination for a strain responsive bridge network mounted on arotating member provided with input and output slip rings, said networkhaving an input and an output connected to said input and output sliprings respectively, comprising a source of alternating current, atransformer having primary, secondary and tertiary windings, saidprimary winding being connected in series between said alternatingcurrent source and one of the input rings to said network, a variableimpedance connected in series with said tertiary winding, said impedanceand said tertiary winding being connected in series across said sourceof alternating current, a second variable impedance connected to saidsecondary winding, a pair of output terminals connected to said outputrings, said secondary winding and said impedance being connected inseries between one of said output rings and one of said outputterminals.

References Cited in the file of this patent" UNITED STATES PATENTSNumber Name Date 2,392,293 Ruge Jan. 1, 1946 2,423,620 Ruge July 8, 19472,447,566 Decker Aug. 24, 1948 2,447,734 Capuzzi Aug. 24, 1948 2,457,165McNamee Dec. 28, 1948

